How should I deal with invalid input to an object constructor in JavaScript

I'm trying to get my head around OOP in Javascript (I've never really used OOP in any other language either, so this is my first experience of it). I've written a class that requires an input for the constructor, but the constructor only only works with a specifically formatted string.

I have a regex that I can use to check the input against, but I don't know what I am supposed to do if it doesn't match (or if there is no input at all). Should I throw an exception of some sort? If so, how do I go about that?

Solution

JavaScript constructors (functions called with the new keyword) always need to return an object, you can't return false, null or similiar like when invoked normal. So, you have two options:

throw an exception from it. You can do so with every javascript value in a throw statement. Then, catch them outside the construction with a try statement
Return a usual object, but with the internal values set to a invalid pointer. The Date constructor does that for instance, getting attributes then results in NaNs.

Additionally (especially when using exceptions) you might provide an extra validation function (which returns boolean) as a static property on the constructor, so you can use that even before constructing an object. Example:

function Decryption(str) {
    if (! Decryption.test(str))
        throw new SyntaxError("invalid encryption");
    this.result = decrypt(str);
}
Decryption.test = function(str) {
    return /MyCoolRegExp/.test(str);
};